What is the authenticity of the hadith: "He who seeks forgiveness has not persisted, even if he returns to it seventy times a day," and how can one act upon it if it is authentic, given the conditions of repentance and the fact that persistence is among the major sins, and how is it not to be understood as "no sin harms with faith"?

The hadith "He who seeks forgiveness is not insistent, even if he returns [to the sin] seventy times a day" is weak. Likewise, the hadith "No major sin with seeking forgiveness, and no minor sin with insistence."

Even if they were authentic, they would be interpreted as referring to the sincere repentance of those who regret their sins and repent, then may fall into sin again and repent anew. Such individuals are not considered persistent [in sin].

The desired form of seeking forgiveness is that which involves both the heart and the tongue, thereby ending persistence [in sin] and achieving remorse. It is, therefore, an embodiment of repentance.

Insistence on a minor sin makes it a major one, and repentance invalidates persistence.

The statement that "sin does not harm with faith" is a saying of the extremist Murji'ah, and it is outright disbelief. It has no connection to seeking forgiveness or repentance.